A Simple Solution (Followup)
Quite a bit of discussion and great ideas in the comments to my previous post about establishing a Fiction tag for fictional websites, and how this could come in really handy for not ruining the immersion into your story world by having a big disclaimer somewhere on the site pulling folks out of the story. In the meantime, the might be an easier temporary solution.
Go to http://unidentifiedenemy.com, which is one of the viral sites for the upcoming Sony Pictures film Battle: Los Angeles. Now, check out the address bar, and what do you see (depending on your browser)? It's a favicon for Sony Pictures. Before the site even finishes loading, this favicon appears, letting me know that this site is related to Sony Pictures.
So yeah, this made me go "hmmmm." What if ARG and transmedia producers simply put some sort of standardized Fiction favicon on their in-universe sites? Something simple, a stylized F or something. It still wouldn't have the functionality of a rollover or something that explains what it is, but it would be a start.
Or, I dunno. Maybe I'm becoming a little obsessed. :P Anyway, something to ponder.